Field of the Invention
This invention relates a low emissivity coating having a high solar heat gain coefficient (SHGC) and a low overall heat transfer coefficient (U factor). The coating is particularly useful for an insulating glass unit (IGU).
Technical Considerations
Conventional architectural window glass is highly thermally emissive. Solar energy easily passes through such glass. In order to reduce the passage of solar energy, low emissivity coatings are applied onto the glass. Low emissivity coatings act as thermal barriers that decrease the emission of radiant infrared (IR) energy, particularly thermal infrared energy. The lower the emissivity, the better the coating is in blocking the emission of thermal IR energy.
The solar heat gain coefficient (SHGC) is a measure of how well the window blocks solar heat. The lower the SHGC, the more solar heat is blocked, i.e., the lower the solar heat buildup inside the building.
The overall heat transfer coefficient (U factor) is a measure of heat loss through the window. The lower the U factor, the lower the heat transfer through the window, i.e. the higher the insulating level of the window.
Conventional low emissivity coatings for architectural windows typically are designed to provide a low SHGC and a low U factor. A low SHGC blocks solar energy, particularly solar infrared energy, passing through the window into the structure. These conventional low emissivity coatings help reduce air conditioning costs in the summer and are well suited for moderate and hot climates.
However, these conventional low emissivity coatings are not well suited for cold climates, such as climates having long winters and short summers or climates having extended periods of very cold weather. In cold climates, the concern for reducing air conditioning costs during the short summer months is much less than the concern for heating the building during the rest of the year. For cold climates, it would be desirable to provide a low emissivity coating that provides a higher SHGC than conventional low emissivity coatings while maintaining a low U factor. A high SHGC allows more solar heat to pass into the building to heat the interior of the building while a low U factor helps to keep the heat inside the building. Optionally, it also would be desirable for such a low emissivity coating to have a high visible light transmittance. High visible light transmittance allows more light to enter the building and decreases the need for lamps and artificial illumination. Optionally, it also would be desirable for such a low emissivity coating to have desirable aesthetics to meet the demands of the residential and commercial markets. These markets tend to desire neutral colors that can be used with a variety of different building colors but still look aesthetically pleasing. Optionally, it also would be desirable if such a coating blocked at least some of the solar ultraviolet (UV) radiation directed toward the coating. Solar UV radiation can damage furniture and can cause fading. Optionally, it also would be desirable for such a coating to block at least some of the long wavelength solar IR energy.